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Here's the thing: AiO coolers are filled with coolant, but not fully - there is a bit of air inside for a couple of reasons (research about it if you want, I'm not going to waste your time explaining here). So when the PC is working, the air bubble naturally finds its way to the highest place in the loop (look at attached picture) - that's how physics work (get a transparent bottle of water and rotate it around for an example).
When the radiator is mounted the way you did it - the air bubble will try to stay right where the tubes are. The problem is that the pump (which is inside the block you mount on top of the CPU) is pulling cold coolant from one of those tubes, and pushing hot coolant through the other. So when you have the air bubble there - the pump will pull air into itself and into the block, making cooling substantially more inefficient.
That is why the recommended mounting options are either having the radiator mounted at the top, or at the front but rotated in a way that the tubes are at the bottom to have a good coolant flow.
Another "solution" (janky as hell) is to just have the PC lying on its side when it's working, so that the air bubble stays on the long side of the radiator, lol.
